Add clear availability notices to platform documentation to reduce user
confusion about app distribution:
- Android: Add notice that pre-built binaries are not yet publicly available
- iOS: Update notice to match Android format and clarify build-from-source option
- Platform index: Add structured breakdown of companion app availability
Changes address user confusion reported in Discord about missing download
links and unavailable pre-built binaries. Users can still build from source
or request early access via Discord.
Fixes#1864
Co-Authored-By: Warp <agent@warp.dev>
